1. Field of the Invention
The invention pertains to systems for treating wastewater and, more particularly, to an apparatus and method for treating wastewater on marine vessels, or land-based systems, so the effluent can safely be released into the environment.
2. Description of the Prior Art
Wastewater generated on marine vessels such as ships, ferries and pleasure craft should be treated before it is released into the marine environment to prevent or reduce environmental contamination. This wastewater can include waste from toilets, sometimes referred to as xe2x80x9cblackwaterxe2x80x9d, waste from showers, sinks, laundry machines, galleys and the like, sometimes referred to asxe2x80x9cgreywater,xe2x80x9d and bilge water. In many jurisdictions, the standards for wastewater effluent discharge from marine vessels is prescribed by government regulation. The standards may limit the discharge of suspended solid, contaminants affecting BOD (biochemical oxygen demand) and pathogens. Hence, effluent treatment may involve suspended solids removal, BOD reduction and disinfection.
Wastewater treatment systems for marine applications typically include the use of disinfecting chemicals such as chlorine or the use of microbiological oxidation. Both systems have disadvantages. Chlorine and similar disinfecting chemicals are themselves environmental contaminants and may form harmful byproducts, such as various chlorinated compounds. They require hazardous chemical storage on the vessel. Their use in marine applications is prohibited in some jurisdictions. Chlorination will only achieve disinfection of the wastewater. Treatment by biological digestion is effective in land-based sewage treatment but is not well suited for marine applications for various reasons. These include the slowness of the process and its sensitivity to influent substances such as surfactants and to changes in salinity, temperature and flow volumes. Biological treatment systems have large footprint requirements. On start-up, time is required for development of the biological growth within the treatment apparatus; this can take several days, resulting in effluent that does not meet discharge limits. The resultant effluent from a biological reactor may still require the removal of suspended solids and disinfection.
It is known that ozone can be used for oxidizing the contaminants in wastewater and for disinfection. For example, U.S. Pat. No. 4,197,200 (Alig) and U.S. Pat. No. 4,053,399 (Donnelly et al.) describe wastewater treatment systems in which ozone gas is employed. However, the use of diffusers to introduce the gas into wastewater will not achieve the micro-bubble size for effective solids removal in dissolved gas flotation nor the efficient reduction of BOD via oxidation of the contaminants. In addition, the diffusers can plug with solids during system shutdown, rendering system maintenance difficult.
The invention provides an apparatus and method for treating wastewater (which can include blackwater, greywater and/or bilge water in marine and land-based applications) that uses ozone as an oxidizing agent and does not rely on microbiological oxidation or involve the use of any additional disinfecting chemicals. The system effectively reduces the BOD, total suspended solids (TSS) and fecal coliform count of the discharged water so it can be released into the environment to meet effluent regulations. The system includes a solids separation tank, an oxidation tank preferably having a plurality of chambers and a fluid circuit for re-circulating treated wastewater into the oxidation tank and, preferably, also into the solids separation tank, for further treatment. Suspended solids are separated by dissolved gas flotation and the wastewater is oxidized by ozone that is dissolved in the treated wastewater (effluent) and re-circulated through the treatment apparatus. Although solids separation takes place mainly in the solids separation tank it will also take place in the oxidation tank. Oxidation takes place in all the chambers. Advantageously, the introduction of ozone into the system as a gas which is dissolved in the effluent and is subsequently released from solution in the reaction vessels via nozzles to form a gas used for flotation and oxidation avoids the use of diffusers to introduce ozone as a gas. A high degree of BOD removal is achieved in this system. The ozone used in the system is generated on the vessel (in the case of marine applications) using electricity produced by the vessel""s engines, so no transporting or storage of tanks of chemicals is required. In the case of difficult to oxidize chemicals such as certain surfactants and oil, advanced oxidation technologies may be included. Advanced oxidation technologies are those that produce hydroxyl radicals which are very aggressive oxidants. One means of producing these radicals is via the exposure of ozone to ultraviolet light.
According to the present invention, there is provided an apparatus for treating wastewater comprising: a wastewater inlet conduit; a solids separation tank to receive wastewater from the inlet conduit, for the separation of solids from liquid in the wastewater; an oxidation tank in fluid communication with the solids separation tank to receive liquid from the solids separation tank; a liquid outlet conduit from the oxidation tank to conduct liquid from the oxidation tank; a source of gas comprising ozone; means for dissolving the gas comprising ozone in liquid from the liquid outlet conduit; a re-circulating circuit for conducting the liquid with dissolved gas comprising ozone to the solids separation tank and the oxidation tank; discharge means for discharging the liquid with dissolved gas comprising ozone into the solids separation tank and oxidation tank, whereby the dissolved gas comprising ozone forms gas bubbles in the solids separation tank and oxidation tank; and a liquid discharge conduit to discharge treated liquid from the apparatus.
In accordance with another aspect of the invention, there is provided an apparatus for treating sewage comprising: a wastewater inlet conduit; a solids separation tank to receive the wastewater from the inlet conduit, with gas distribution means therein for the separation of solids from liquid in the wastewater by gas flotation; an oxidation tank in fluid communication with the solids separation tank to receive liquid from the solids separation tank; a liquid outlet conduit from the oxidation tank to conduct liquid from the oxidation tank; a source of gas comprising ozone; means for dissolving the gas comprising ozone in liquid from the liquid outlet conduit; a re-circulating circuit for conducting the liquid with dissolved gas comprising ozone to the oxidation tank; discharge means for discharging the liquid with dissolved gas comprising ozone into the oxidation tank, whereby the dissolved gas comprising ozone forms gas bubbles in the oxidation tank; and a liquid discharge conduit to discharge treated liquid from the apparatus.
In accordance with yet another aspect of the invention, there is provided a method for treating wastewater comprising the steps of: providing a treatment system comprising a solids separation tank, an oxidation tank and a liquid flow circuit whereby liquid flows from the solids separation tank into the oxidation tank, out of the oxidation tank and is reintroduced into the solids separation tank and the oxidation tank; dissolving a gas comprising ozone into the liquid in the liquid flow circuit after the liquid exits from the oxidation tank; introducing wastewater to be treated into the solids separation tank; separating solids from liquid in the wastewater in the solids separation tank; allowing the liquid from the solids separation tank to pass into the oxidation tank; introducing the liquid with dissolved gas comprising ozone into the liquid in the oxidation tank and allowing the gas comprising ozone to form bubbles in the liquid in the oxidation tank and cause oxidation of substances in the liquid in the oxidation tank; and removing treated liquid from the treatment system for discharge to the environment.
In accordance with yet another aspect of the invention, there is provided a method for treating wastewater comprising the steps of: providing a treatment system comprising a solids separation tank, an oxidation tank and a liquid flow circuit whereby liquid flows from the solids separation tank into the oxidation tank, out of the oxidation tank and is reintroduced into the oxidation tank; dissolving a gas comprising ozone into the liquid in the liquid flow circuit after the liquid exits from the oxidation tank; introducing wastewater to be treated into the solids separation tank; separating solids from liquid in the wastewater in the solids separation tank by means of gas flotation; allowing the liquid from the solids separation tank to pass into the oxidation tank; introducing the liquid, with dissolved gas comprising ozone into the liquid in the oxidation tank and allowing the gas comprising ozone to form bubbles in the liquid in the oxidation tank and cause oxidation of substances in the liquid in the oxidation tank; and removing treated liquid from the treatment system for discharge to the environment.